Article pubs.acs.org/Macromolecules
Direct Amination of Polyethylene by Metal-Free Reaction Houbo Zhou,† Shuangshuang Wang,‡ Huahua Huang,† Zhiyong Li,† Christopher M. Plummer,† Shaoli Wang,‡ Wen-Hua Sun,‡ and Yongming Chen*,† †
School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
‡
S Supporting Information *
ABSTRACT: Using recently developed alkane C−H amination chemistry, a mild postmodification of polyethylene (PE) to form C−N grafts without using a metal catalyst is reported. This method applies N-hydroxyphthalimide (NHPI) as an organic catalyst to cleave C−H bonds to generate a carbon radical that reacts with the NN of dialkyl azodicarboxylates to form a hydrazine unit. The amination of linear PE was successfully conducted with bis(2,2,2-trichloroethyl)azodicarboxylate (BTCEAD) and di-tert-butyl azodicarboxylate (DBAD) in the presence of NHPI in tetrachloroethane at 110 °C. Both BTCEAD and DBAD units have been introduced into PE chains. By optimizing the conditions, like time and feed ratio, the number of grafted BTCEAD units can reach about 10 per 100 ethylene units. Moreover, the DBAD units of PE may be transformed into hydrazine units by treating with trifluoroacetic acid (TFA). The PE modified by BTCEAD and DBAD grafts was blended with poly(methyl methacrylate) (PMMA) and the compatibility was greatly improved. Direct modification of a PE membrane by treating with a solution was also successful.
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INTRODUCTION Polyolefins are currently one of the most important synthetic organic materials.1−3 They are produced by olefin polymerization catalyzed by transition metal catalysts.4 However, polyolefins are hydrophobic, and thus it is difficult to blend them with other polymers to improve their properties.5 It is also hard to print or color the surface of polyolefin materials due to a lack of functional groups. Therefore, functionalization of polyolefins is a very important issue for their application.6−8 Although it is possible to introduce hydrophilic components by catalytic copolymerization of olefins and hydrophilic/polar monomers,4,9−11 it is very difficult to make this production practical due to either catalytic activity problems or the cost of the technology translation. Presently, industrial functionalization of polyolefin is mainly based upon free radical grafting of maleic anhydride (MAn) along polymer backbones.12−14 By this method, polymers are treated with large amounts of peroxides and MAn at high temperatures. The C−H bonds of the polymer are cleaved by the free radicals generated from peroxides, giving carbon radicals that then add to the double bond of MAn. The grafted anhydride may then be transformed to other functionalities by reaction with nucleophilic chemicals.15 Additionally, azide reagents, like aromatic azides16 and sulfonyl azides,17,18 are used to modify polyolefins via the nitrene intermediate, thus leading to the introduction of C−N bonds into the polymer backbones. However, all the above methods suffer serious side reactions such as the cleavage and cross-linking of polymer chains due to radical properties.2,19 Also, they have problems relating to the use of either toxic and corrosive MAn and explosive peroxides or azides. Therefore, alternative environ© XXXX American Chemical Society
mentally friendly methodologies for modifying polyolefins have been explored for a few decades. Many groups are exploring new methodologies to postmodify polyolefin chains using different chemistry. Hillmyer et al. reported that rhodium-catalyzed alkane borylations may be used to activate the primary C−H branches of polyethylethylene (PEE), with a subsequent oxidation introducing polar OH groups into the PEE.8 This chemistry was conducted in the presence of catalytic [Cp*RhCl2]2 and a diboron reagent at a temperature of 150−200 °C. It has also been applied to modify polypropylene (PP)20 and low-density polyethylene (LDPE).21 Direct oxyfunctionalization of poly(ethylene-alt-propylene) using a manganese complex was reported, and tertiary alcohol groups were thus introduced.22 Perez et al. reported that poly(1-butane) may be functionalized with ester groups by a copper-based catalytic insertion of CHCO2Et into C−H.23 Recently, the same group applied this chemistry to modify poly(styrene-co-butadiene).24 In this case, the carbene group from ethyl diazoacetate is the critical active group that inserts into either saturated polyolefins or carbon−carbon double bonds of unsaturated polyolefins.25 However, the chemistry for postmodification of polyolefins under mild condition remains a big challenge. Moreover, previous attempts are mainly based upon metal catalysts, and transformation efficiency is normally low. In the past decade, C−H bond activation has become a revolutionary process in organic chemistry.26−28 It allows Received: November 30, 2016 Revised: April 5, 2017
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DOI: 10.1021/acs.macromol.6b02572 Macromolecules XXXX, XXX, XXX−XXX
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samples by elemental analysis. As shown in Table 1 the grafting of BTCEAD, nBTCEAD, defined as the number of grafted units
coupling of chemicals via C−H bonds or directly transferring C−H bonds of simple chemicals to useful functional groups. It also shows very promising applications for polymer modification. Activation of C(sp3)−H is a crucial step in the modification of saturated polyolefins. Therefore, achievements in new organic reactions could be very useful to develop new strategies to functionalize polyolefins. Recently, Inoue et al. reported a direct conversion of C(sp3)−H to C(sp3)−N using N-hydroxyphthalimide (NHPI)29 as a catalyst to cleave the C− H bonds.30 Oxidation of the NHPI generates the electrondeficient phthalimide N-oxyl radical (PINO), which abstracts hydrogen to give a carbon radical. When dialkyl azodicarboxylate (DAAD) is present, the carbon radical is added to DAAD, forming hydrazine derivatives. It was shown that this reaction occurs under mild conditions. Substrates like cyclododecane can be modified by introducing hydrazine in 87% yield at 80 °C. This result prompted us to explore its application for introducing C−N along the backbone of polyethylene (PE). PE is a very important polymer material, and unfortunately, its postfunctionalization is difficult to perform since it lacks either relatively more reactive tertiary C−H or primary C−H atoms.31 We chose PE as the substrate to prove the feasibility of the chemistry of direct amination by NHPI. As shown in Scheme 1, the reagents BTCEAD and DBAD were chosen to
Table 1. Functionalization of PE with BTCEAD Catalyzed by NHPIa entry
[E100]/ [BTCEAD]b
t (h)
C/Nc
gd (wt %)
nBTCEADe
1 2 3 4 5 6 7 8 9 10
1:2.4 1:2.4 1:2.4 1:2.4 1:2.4 1:5.6 1:17 1:50 1:83 1:170
0.25 0.5 1 2 4 1 1 1 1 1
1700 667 70.0 69.0 71.4 56.3 31.6 16.6 11.9 11.2
0.7 1.7 14 15 15 18 29 45 56 58
0.05 0.13 1.3 1.3 1.3 1.6 3.0 6.1 9.2 9.9
efficiencyf (%)
54
29 18 12 11 6
a
NHPI: 20 wt %, relative to PE; tetrachloroethane: 0.6 M, based on repeat units of PE; 110 °C. bMolar ratio of 100 ethylene units to BTCEAD. cCarbon-to-nitrogen ratio by elemental analysis. dGrafting content of BTCEAD. eNumber of BTCEAD grafted to 100 ethylene units. fGrafting efficiency relative to the diazocarboxylates.
per 100 ethylene units, increased gradually and reached a maximum of 1.3 in 1 h. Then the effect of the feed ratio of [E100]:[BTCEAD] on nBTCEAD grafting during 1 h was examined, and the results collected in Table 1. It was discovered that the nBTCEAD increased with the feeding of BTCEAD. At [E100]:[BTCEAD] = 1:170, the nBTCEAD reached 9.9, meaning that there are 9.9 BTCEAD units introduced into every 100 ethylene units. This is remarkably high for polyolefin modification. Because of the introduced BTCEAD units with a relatively high molecular weight, the increased mass of the modified PE was very obvious, as given by grafting content g shown in Table 1. Conversion efficiency of BTCEAD was as high as 54% with a low feed of [E100]:[BTCEAD]. However, it was found that the conversion efficiency became low with an increased ratio of BTCEAD (Table 1). Similarly, for the reaction with DBAD under different ratios [E100]:[DBAD] with 20 wt % NHPI, a maximum was reached in 1 h, and with an increase of [DBAD], the nDBAD increased up to 2.1 Table 2. The amount of incorporated DBAD units is lower than that of BTCEAD, which matched the results reported for small molecular substrates.30 This may be attributed to a relatively higher activity of BTCEAD in radical addition owing to the electronwithdrawing effects of the trichloroethyl group. The introduction of BTCEAD units into PE was confirmed by FT-IR, 1H NMR, and 13C NMR. In contrast to neat PE that has no absorption at carbonyl stretching frequencies, the product of entry 3 in Table 1 with nBTCEAD = 1.3 gave a strong peak of vCO at 1735 cm−1 as shown in Figure 1. The FT-IR spectra of BTCEAD and DBAD were measured (Figure S1), and the vCO of the unreacted BTCEAD was at 1784 cm−1, indicating that the observed peak in Figure 1 is not due the presence of free BTCEAD within the polymer. Moreover, a strong peak of vCO at 1710 cm−1 was found from the modified PE with DBAD (Figure 1C). It also differs to that of free DBAD, indicating the presence of chemically tethered DBAD units. In the 1H NMR spectrum of the PE-g-BTCEAD (Figure 2A), the proton of the PE carbon being modified gave a resonance at 4.17 ppm due to the deshielding effect of the neighboring
Scheme 1. Schematic Illustration of the Functionalization of PE with NHPI/BTCEAD and NHPI/DBAD by C(sp3)−H Activation
be grafted to PE. On one hand, the introduced fragments may change the polarity of PE, and on the other hand, they may be transformed to amines which are very useful for PE applications.
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RESULTS AND DISCUSSION Modification of PE in Solution. A linear PE sample, Mn 7.9 kDa and PDI 1.83, given by size exclusion chromatography (SEC), was selected as a proof of concept. This polymer was prepared using a new catalytic cobalt system developed by our coauthors.32 The PE obtained by this system shows a relatively low polydispersity and a highly linear structure. According to the literature protocol for small molecular substrates,30 experiments were first conducted under conditions of 110 °C, a feed ratio of [E100]:[BTCEAD] = 1:2.4 (E is ethylene unit), in the presence of 20 wt % NHPI relative to PE, and in tetrachloroethane (TCE) for various reaction times. After the reaction, the polymer was precipitated in methanol three times to remove any small molecules present. After drying, the samples were characterized by elemental analysis to obtain carbon and nitrogen content. First, the reaction time was optimized by monitoring the C/N ratio of the modified B
DOI: 10.1021/acs.macromol.6b02572 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Functionalization of PE with DBAD Catalyzed by NHPIa entry
[E100]/ [DBAD]b
t (h)
C/Nc
gd (wt %)
nDBADe
efficiencyf (%)
1 2 3 4 5 6 7
1:5.6 1:8.3 1:17 1:34 1:50 1:83 1:131
1 1 1 1 1 1 1
224 230 167 83.8 51.6 48.8 45.3
3.1 3.0 4.2 8.2 13 14 15
0.39 0.38 0.52 1.1 1.8 1.9 2.1
7.0 4.6 3.1 3.2 3.6 2.3 1.6
a
NHPI: 20 wt %, relative to PE; tetrachloroethane: 1.2 M, based on repeat units of PE; 110 °C. bMolar ratio of 100 ethylene units to DBAD. cCarbon-to-nitrogen ratio by elemental analysis. dGrafting content of DBAD. eNumber of DBAD grafted to 100 ethylene units. f Grafting efficiency relative to the diazocarboxylates.
Figure 3. 13C NMR (o-C6D4Cl2/o-C6H4Cl2, 300 MHz, 110 °C) spectra of (A) PE-g-BTCEAD (entry 8, Table 1) and (B) PE-g-DBAD (entry 6, Table 2).
ppm, respectively. Moreover, carbons 6 (PE), 8, 9, and 10 (DBAD) were assigned at 58, 155, 80, and 28 ppm from the 13 C NMR spectra of PE-g-BDAD, respectively (Figure 3B). Therefore, it has been unequivocally proven that BTCEAD and DBAD units were chemically bonded to the PE chains by treating the parent material with BTCEAD and DBAD with the aid of NHPI. It is known that the modification of polyolefins by free radical methodologies often faces problems of side reactions at high temperatures. In the case of PP, chain scission due to free radical induced rearrangement competes with functionalization, while in PE cross-linking due to radical−radical coupling is observed.2 The modified PE samples in this study were characterized by SEC. As shown in Figure 4, the traces of PE-g-
Figure 1. FT-IR spectra of (A) original PE, (B) PE-g-BTCEAD (entry 3, Table 1), and (C) PE-g-DBAD (entry 6, Table 2).
Figure 4. SEC traces and characterization results of (A) PE-gBTCEAD (entry 9, Table 1), (B) PE-g-DBAD (entry 6, Table 2), and (C) original PE with 1,2,4-trichlorobenzene as eluent at 150 °C using polystyrene standards for calibration.
Figure 2. 1H NMR (o-C6D4Cl2, 400 MHz, 110 °C) spectra of (A) PEg-BTCEAD (entry 8, Table 1) and (B) PE-g-DBAD (entry 6, Table 2).
nitrogen. The resonances at 4.68 and 6.41 ppm are attributed to the protons of CH2O and NNH, respectively. In terms of PE-g-DBAD, the proton resonance of the CH(PE)N and NNH can be found at 4.07 and 5.79 ppm as shown in Figure 2B. Moreover, the CH3 of the tertiary butyl moiety gave an expected response at 1.42 ppm. The 1H NMR spectra of other modified PEs with different grafting contents are offered in the Supporting Information (Figures S6 and S7) and also confirm their structures. Correspondingly, from the 13C NMR spectrum (Figure 3A) of PE-g-BTCEAD, it was discovered that the resonance of the carbon 1 atom bonded to the nitrogen atom was located at 60 ppm, and the resonances of carbons 3, 4, and 5 within the BTCEAD unit were located at 154, 75, and 95
BTCEAD and PE-g-DBAD were consistent with that of the parent PE. The PDI values of all modified samples are less than 2 (Table S1). Therefore, the present chemistry does not alter the PE chain properties, demonstrating that the conditions of the modification are mild. Thermal Analysis. The results demonstrate promising amination chemistry for PE postmodification. The introduced units may alter the properties of the PE material. Thermal analysis of two different modified PE samples was conducted. The results of thermal gravimetric analysis (TGA) of samples with differing graft content of BTCEAD and DBAD are given in Figures 5A and 5B, respectively. In comparison with the C
DOI: 10.1021/acs.macromol.6b02572 Macromolecules XXXX, XXX, XXX−XXX
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Surface Modification. Improving the hydrophilicity of polyolefin materials is a very important aspect of their application.33,34 In the above section, it was proved that PE modification by NHPI is efficient in solution. Functionalization of the surface of a solid PE film by direct treatment with a solution of NHPI and azodicarboxylate would be more useful for practical applications. Therefore, we explored a direct surface modification of a PE film by this chemistry. PE films were immersed in a dichloroethane (DCE) solution of NHPI and azodicarboxylates at 80 °C for 24 h. After washing and drying, the films were analyzed by FT-IR. Figure 6 shows the
Figure 5. TGA curves of (A) PE-g-BTCEAD and (B) PE-g-DBAD with different grafting contents.
Figure 6. FT-IR spectra of (A) original PE membrane, (B) PE membrane grafted DBAD, and (C) grafted BTCEAD.
parent PE, the modified samples show two steps of degradation. These two stages of degradation for the modified PE samples occurred at ca. 220 and 250 °C; the weight loss increased with an increase in graft content. The percentages of weight loss were close to the grafting contents (g, wt %) calculated by elemental analysis. So it was expected that this step could be attributed to the thermal degradation of BTCEAD and DBAD units from the backbone. The second decomposition step of the two samples was very similar to the unfunctionalized PE. Thus, this stage was believed to be a property of the PE main chains. Differential scanning calorimetry (DSC) was applied to measure the effect of the introduced units. The melting temperature (Tm) and crystallinity (Xc) of two modified PE samples were collected in Table 3, and typical DSC curves are
absorption spectra of PE membranes modified by BTCEAD and DBAD, respectively. Compared to the parent film, there appeared vCO absorptions at 1730 and 1713 cm−1 from the two samples, which correctly matched the results of the solution modification as shown in Figure 1. Moreover, we measured the contact angle of the modified PE films. The static contact of the parent PE film is 102.2 ± 1.4°. As a contrast, the contact angle of the PE films modified by BTCEAD and DBAD dropped to 84.5 ± 3.6° and 90.5 ± 2.7°, respectively, implying that the introduced fragments increase the hydrophilicity of the polymers. Therefore, the modification chemistry is also efficient in the functionalization of the surface of PE films. Blending with PMMA. It is known that neat PE is unlikely to be compatible with other polymeric materials.35 Successful functionalization of PE with either BTCEAD or DBAD prompted us to blend the modified PE with other polymers to check for any miscibility improvement. Herein, we dissolved poly(methyl methacrylate) (PMMA) and PE-g-BTCEAD or PE-g-DBAD in the same solvent (toluene). Then blend films of two samples at a weight ratio of 80:20 (PE or modified PE:PMMA) were obtained by evaporation of the solvent. In order to observe the phase separation structure, the films were immersed into acetone to etch the PMMA phase away. The remaining solid materials were then observed by scanning electron microscope (SEM). As shown in Figure 7A, the film from the parent PE/PMMA shows a porous morphology with a pore size over 10 μm, demonstrating that the two polymers are not compatible. When the PE-g-BTCEAD was blended with PMMA, the PMMA phase size became much smaller (Figure 7B), and with an increase in the graft content, the size of the PMMA phase decreased to a few hundred nanometers (Figure 7C). For the case of PE-g-DBAD, the PMMA phase size also became smaller (Figure 7D). Therefore, the functionalization may greatly improve the compatibility of PE with PMMA. It is noteworthy that PE-g-BTCEAD showed better compatibility
Table 3. DSC Characterization Results of PE-g-BTCEAD and PE-g-DBAD with Different Grafting Contents run no.
polymer
Tm (°C)
Xc (%)
1 2a 3 4 5b 6
PE PE-g-BTCEAD PE-g-BTCEAD PE-g-BTCEAD PE-g-DBAD PE-g-DBAD
130 119 108 90 123 116
70 29 18 9 47 32
a
Runs 2−4 came from entries 6−8 of Table 1. bRuns 5 and 6 came from entries 3 and 6 of Table 2.
shown in Figure S8. After modification, PE derivatives were still crystalline, but their Tm and Xc decreased with an increase in grafted fragments. This is due to the influence of randomly grafted moieties on chain folding, leading to crystalline imperfection of modified PE. Thus, the results of thermal analysis further prove the successful molecular modification of PE chains. D
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precursor PE-g-DBAD (116 °C, 32%). This is reasonable because the size of grafted segments becomes much smaller owing to removal of Boc group (Figure S5).
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CONCLUSION We have shown that the C−H bonds of PE chains may be activated and cleaved by NHPI. By addition to azodicarboxylates, C−N bonds are formed along the PE chains. As far as we know, this is one of the few examples where PE chains are functionalized by the direct introduction of C−N bonds. The reaction was conducted at 110 °C in tetrachloroethane in the presence of 20 wt % NHPI to catalyze C−H cleavage, using an azodicarboxylate to react with the carbon radicals generated. Regarding the quantity of graft units, as many as 10 per 100 ethylene units can be introduced. Interestingly, it was also discovered that the modification reaction can be carried out on the surface of a solid PE membrane. This discovery will certainly stimulate further interest to find applications of PE membranes for direct printing and for improving interfacial adhesion with other materials.
Figure 7. SEM images of PMMA-etched blend fractured surfaces. (A) PE/PMMA, (B) PE-g-BTCEAD (entry 7, Table 1)/PMMA, (C) PE-gBTCEAD (entry 8, Table 1)/PMMA, and (D) PE-g-DBAD (entry 7, Table 2)/PMMA. Weight ratio: PE or modified PE:PMMA = 80/20.
than PE-g-DBAD. Thus, BTCEAD units are more likely compatible with PMMA than DBAD. Hydrazine-Modified PE. PE-g-DBAD is actually the PE derivative of tert-butoxycarbonyl (Boc) protected alkylhydrazine. Therefore, we explored the removal of Boc by treating PEg-DBAD with trifluoroacetic acid (TFA) in TCE at 110 °C under nitrogen protection (Scheme 2).36 A complete removal
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02572. Experimental procedures, spectra and spectroscopic data (PDF)
Scheme 2. Schematic Illustration of the Deprotection of PEg-DBAD To Give PE-g-NHNH2
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AUTHOR INFORMATION
Corresponding Author
*(Y.C.) E-mail:
[email protected]. ORCID
of Boc was confirmed by FT-IR, 1H NMR, and 13C NMR analysis. As shown in Figure S2, the vCO at 1710 cm−1 of PEg-DBAD disappeared. In addition, a difference in the resonance of the proton located at 2.79 ppm, attributed to the CH adjacent to the NHNH2 moiety, as well as the disappearance of the Boc group signal at 1.42 ppm provided further confirmation (Figure 8). This fact was also readily confirmed by comparisons of the 13C NMR spectra (Figure S3). The SEC trace of PE-gNHNH2 was roughly the same as that of PE-g-DBAD which indicated efficient transformation (Figure S4). Moreover, both the Tm (126 °C) and Xc (58%) increased relatively to its
Wen-Hua Sun: 0000-0002-6614-9284 Yongming Chen: 0000-0003-2843-5543 Author Contributions
H.Z. and S.W. made equal contributions. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from Natural Science Foundation of China (No. 51533009, 21090350), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013S086), and Natural Science Foundation of Guangdong Province (No. 2014A030312018) is gratefully acknowledged.
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DOI: 10.1021/acs.macromol.6b02572 Macromolecules XXXX, XXX, XXX−XXX
